1. Field of the Invention
The present invention relates to an observation optical system, and more particularly to an optical system adapted for use in a device called head-up display or spectacle-type display.
2. Related Background Art
Conventionally there have been proposed display devices in which a cathode ray tube (CRT) or a liquid crystal display (LCD) is positioned close to the head of the observer whereby the observer is enabled to observe the image formed by such CRT or LCD, as disclosed for example in U.S. Pat. Nos. 4,081,209 and 4,969,724 and Japanese Patent Laid-Open Application Nos. 58-78116, 2-297516 and 3-101709. Japanese Patent Laid-Open Application No. 3-101709 discloses an observation device providing a relatively easily observable real image by re-focusing an original image. However, a considerably large space is unavoidable because an optical lens is employed for re-imaging.
On the other hand, U.S. Pat. Nos. 4,081,209 and 4,969,724 and Japanese Patent Laid-Open Application Nos. 58-78116 and 2-297516 disclose observation devices designed to observe a false image which is advantageous for making the device compact though it is somewhat inferior in the ease of observation.
Though the observation device of the latter type can achieve compaction in comparison with that of the real image type, the extent of such compaction cannot be said sufficient. Among the above-mentioned prior technologies, the one disclosed in the Japanese Patent Laid-Open Application No. 58-78116 is relatively advanced in terms of compaction, but the device still has a large thickness in the direction of the axis of the eye. It is also described that the observed image involves optical distortion, astigmatism and coma. A related technology is also described in U.S. patent application Ser. No. 08/317,528 filed Oct. 4, 1994.